The Sudoku puzzle was originally designed by Howard Gains, a retired architect and freelance puzzle maker and was first published in 1979. The puzzle was inspired by an eighteenth century invention of a Swiss mathematician Leonard Euler. Euler's Latin Square was a table or grid filled with symbols such that each symbol occurs exactly once in each row and column. Gains presented the Latin Square as a puzzle by providing a partially completed grid which then required the solver to fill in the rest of the numbers. The puzzle was first published in New York by Dell Magazines under the title “Number Place.”
The puzzle reappeared in Japan in the paper Monthly Nikolist in April 1984 as Suuji W A Dokushin Ni Kaguru which translates as “the numbers must be single.” The name was abbreviated to Sudodu. Within Japan, Nikoli Publishers still hold the trademark for the name Sudoku. In 1989, Loadstar/Softdisk Publishing unveiled a computerized-version of the game for the home computer. Other computer versions for the Apple® and the Palm Pilot® also appeared in the mid-nineties. In 1997, a New Zealander named Wayne Gould began to develop a computer program to produce Sudoku puzzles quickly. He promoted Sudo Ku to the London Times which launched it on Nov. 12, 2004. Since then, Sudoku has appeared in numerous printed versions such as newspapers and puzzle books as well as electronic versions that may be played on personal computers, cell phones, the internet, handheld units and the like. Also, in 2005, Sky Sports in England introduced Sudoku Live which was a game show that used pencil and paper to solve standard puzzles in a team format. Sudoku has become an international sensation and has been called “the Rubik's cube of the 21st Century.”
In a typical Sudoku puzzle, not only numbers, but any set of distinct symbols can be used in the grid. Letters, shapes, colors, and any other collection of nine symbols can be used as the solution set. The objective of the puzzle is to complete a 9×9 grid that consists of nine 3×3 boxes so that each column, row and each box contains the digits 1 through 9 only once. Typical Sudoku terminology describes a grid consisting of 9 boxes each of which contains 9 cells. Variations in grid size have been introduced, although the 9×9 grid is traditional and most common. Variants such as 12×12, 16×16, and 25×25 have appeared in various publications. Sudoku puzzles can be ranked by the degree of difficulty of their solution. Typically, the levels are “easy”, “difficult”, or “hard”. The level of difficulty is driven by the positioning and relationships between the given numbers not necessarily by the quantity of numbers given. The attraction of the puzzle is that the completion rules are simple, yet the reasoning to reach a solution may be difficult. A so-called “well-formed” Sudoku puzzle has one and only one solution.